1. Field of the Invention
The present invention is directed to a deice for measuring rotary speed of a rotating member, and more particularly to such a rotary speed measuring device employing a sensor coil which develops a magnetic field and which is disposed in the vicinity of a rotating wheel for monitoring an inductance change of the coil caused by regularly appearing marks on the periphery of the wheel, the inductance change being evaluated with a predetermined threshold to give a pulse for giving the rotary speed of the wheel.
2. Description of the Prior Art
U.S. Pat. No. 4,574,237 discloses a prior rotary speed measuring device with the use of a sensor coil conducting an AC current to develop therearound a magnetic field in which a rotating wheel is disposed. The rotating wheel is formed along its circumference with evenly spaced first marks alternated by second marks of different magnetic property from the first marks. As the wheel rotates, the magnetic field of the coil is influenced by successively approaching first and second marks so as to result in a corresponding change in the inductance of the coil. The inductance change of the coil is given in terms of a voltage across the coil, which voltage is then compared with a predetermined threshold to give a pulse indicative of that one first mark is passed by, i.e., the wheel is rotated by one step. A counter is provided to count the pulse within a unit time to give a rotary speed of the wheel. In view of that the voltage developed across the coil (hereinafter referred to as "coil voltage") will decrease as the coil is spaced far from the wheel, the prior art device relying solely upon the coil voltage itself suffers from a restriction that the threshold should vary to a large extend as the distance between the wheel and the coil varies, in order to effect successful comparison between the coil voltage and the threshold. In other words, when the coil is offset from a predetermined standard position, the threshold should be set to a largely different value from a predetermine standard value. Otherwise, it would be possible that the device fail to recognize the rotation of the wheel even if the coil voltage varies in response to the rotary motion of the wheel. This reduce design flexibility and makes it difficult to correctly measure the rotary speed of the wheel, particularly when the coil is spaced far from the wheel.